The invention relates to a radar system of the frequency-modulated type, supplying to a transmit aerial a continuous wave whose frequency f varies almost linearly between a lower frequency f.sub.1 and a higher frequency f.sub.2 so that f.sub.2 -f.sub.1 =.DELTA.f, comprising a homodyne receiver linked to a receive aerial for supplying a first beat signal Fb.sub.1 between transmitted waves and waves received after reflection, suitable for measuring altitude h by measuring the overall phase rotation (counting the number of axis crossings) of the said beat signal Fb.sub.1 for the duration of .DELTA.t of each sweep .DELTA.f of the transmitted frequency.
When a radio altimeter is realised, intended to be installed aboard an aircraft, it is generally desired to measure altitudes over a very wide range and, possibly, running from zero altitude to an altitude upwards of 10,000 m. Currently two methods are frequently used for realising a radar altimeter. The first method consists of measuring directly the time of travel of a radio pulse or a pulse train encoded according to a code for example of the PN type. This type of equipment operates well at high altitudes but becomes critical when required for measuring very low altitudes as a result of the not insignificant width of the pulses and of the various swaying movements. The other method, which is used according to this invention, consists of measuring indirectly the time of travel by means of the beat frequency obtained from the correlation between a transmitted wave, linearly frequency modulated, and the signal received from the ground. This type of device called FM/CW, at least when it comprises two separate aerials, one for transmission and the other for reception, as is the case for this invention, is better adapted than the above pulse radar for measuring low altitudes but causes problems of insufficient precision and, above all, limitation when measuring high altitudes. In the latter case in fact, the echo signal received from the ground is very weak and in order that the beat signal can always be identified, the linearity in modulation of the transmitted signal should generally be very good and the phase jitter of the beat signal negligible. At high altitudes the FM/CW radar altimeter is limited from the moment when the phase jitter of the beat signal between transmission and reception caused by noise and other non-linearities is of the order of 2.pi..
The echo signal received from the ground is the sum of a large number of independent echo signals but in which the phase near the foot of the straight line passing through the altimeter and having a perpendicular position relative to the ground dominates. This sum which fluctuates considerably and can be represented by a vector OS can resolve itself in the sum, in the phase plane, of a slowly fluctuating mean vector OA and a random sum of vectors moving over a range which, with respect to the modulus of the vector OA, for a given material is larger according as the altitude is larger. The main portion of the beat spectrum of the frequency-modulated altimeters is connected with the mean vector OA. Supposing an altimeter which, at the instant t.sub.1 at which the transmitted frequency is f.sub.1, is situated at a distance h from the mean point A corresponding with the main ground echo signal and which provides the above mean vector OA. The phase difference .phi..sub.1 between the transmitted signal and the received signal can then be written as: ##EQU1## with: c: velocity of the electromagnetic wave
.phi.: fixed angle depending on the altimeter circuitry.
Similarly, at an instant t.sub.2 very near to t.sub.1, for the frequency f.sub.2, while the altitude h has not varied between the mutually adjacent instants t.sub.1 and t.sub.2 : ##EQU2## From the equations (1) and (2) the relationship of the frequency-modulated altimeters is derived: ##EQU3##
The equation (3) is better known in its derived form with respect to time: ##EQU4## where fb denotes the frequency of the above beat signal Fb.
The equation (3) can be directly used in zero-count altimeters as is the case for the present invention. The equation (3), assuming that .DELTA..phi.=k.pi. (k=integer) can in fact be written as: ##EQU5## Such altimeters of the FM/CW radar type are known specifically from the Radar Handbook, Skolnik, published in 1970 by McGraw-Hill, Chapter 16-18, specifically page 16-29. Let us assume that one of these conventional altimeters whose receive means effect only a single frequency change, that is to say, supply only a single beat signal Fb.sub.1. This signal Fb.sub.1 can be represented by the above vector OS (FIG. 1). If the local oscillation vector O'O, representing the abscissa axis in the phase plane, has an amplitude which is higher than that of the vector OS, the beat signal is thus practically the projection of the vector OS on the local oscillation vector O'O. The point A in the phase plane describes, in substance, a circle and the point S describes parasitic random undulations relative to this circle which undulations may or may not constitute loops. At the crossing of the coordinate axes of the phase plane these undulations may be represented by a plurality of zeros, at the crossing of each axis, instead of a single zero, more specifically, by a single or various pairs of additional parasitic zeros. This occurs when the beat signal is affected by considerable phase jitter. A zero-count FM/CW altimeter as indicated above would not differentiate parasitic pairs of zeros and would indicate too high a number of zeros, or too large a measured altitude, in accordance with equation (5). Alternatively, FM/CW radio altimeters are known operating with controlled beat frequency signals. In the latter case the frequency of the beat signal is maintained at a substantially constant value and the above parasitic random undulations are less distinct, that is to say, that this type of equipment is more resistant to jitter, but becomes ineffective in its turn if the phase errors become more important to the extent that this results in the systematic formation of loops concerning parasitic random undulations, in which case the equation (4), which is used for determining the altitude h, can no longer be used.
In all cases, between the instants t.sub.1 and t.sub.2 at which the frequencies are f.sub.1 and f.sub.2 respectively, whatever the frequency curve f(t), the phases of the vector OS exist and do not depend on intermediate values.